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Yanmar 3GMF Hard Starting - Air, Oil, or Fuel Pump?

Jan 13, 2015
Hunter 34 Deep Bay, BC
I bought a 1983 Hunter 34 with the original Yanmar 3GMF diesel in January 2015. There is no hour meter on the motor. From the time I got it until IT happened it started right away (maybe 5 revolutions and off we go). Then I decided to maintain it...

Read carefully, because there's a quiz at the end. No math, though.

I had periodically checked the oil for level, but not examined it closely. Not knowing for sure how long it had been, I changed it. The oil that came out was very, very thick and black, and I could only get about 1.5 litres of the nominal 2 litre capacity out. I filled it with Rotella 15W40 synthetic, ran it for half an hour, then emptied and refilled it. I did this twice more (for a total of three fills) until the oil that came out looked reasonable. Not perfectly clear, but looking much more like new oil than used oil. So far, so good.

Then I decided to check the secondary fuel filter. It looked perfect, so I replaced it (using the same O-ring). Idiotically following the advice of "a guy" I did not bleed it before starting (supposedly the engine would just "swallow" the air). It started, ran for 5 minutes, and quit. I could not get it restarted that day.

The next day I went down and bled both the filter and the high-pressure pump, using the squeeze bulb at the tank, because I could get nothing to happen with the little lever at the fuel pump. I now know about the cam position and lifting the lever hard all the way; more on that later. It took quite a bit of cranking, but it started. Once it was running, it ran smoothly with no visible smoke (but again, more on that later). The day following that I got it started after a half a minute to a minute, and took it for a three hour run at 2800 rpm. It ran perfectly the whole time.

Since that time I've started it 16 times (by my log). When it's cold out, which around here is maybe 0 - 5°C (32 - 40°F) it might take a minute or more to get it to fire. Now that it's warming up (5-12°C - 40-55°F), it seems to take roughly 30 seconds to start. Given the relationship between dates and temperature, it could also be that things are improving with time, not temperature.

While cranking, there can be a hint of white smoke. When it fires, there is a cloud about 5' in diameter of very white smoke, together with a hint of oil sheen on the water, that immediately dissipates and from that point on there is no visible smoke - and it runs perfectly smoothly.

I have repeatedly bled it at both the filter and the H-P pump. I now know that I can get something from the fuel pump with the manual lever if I push hard and occasionally I have to rotate the crankshaft slightly to move the cam before anything happens. But the most I ever get is a drop of diesel fuel, maybe 1/8" across. This seems pretty paltry to me.

At the suggestion of a Yanmar mechanic (who I drove an hour to go see, because you don't want to pay for him to come to the boat, just trust me on that) I checked and reseated the O-ring on the fuel filter (stupidly not changing the d***d thing - I'll do that next). His comment was that if the engine ran for 10 minutes there could not be any residual air in the system, and if you recall, I ran it for 3 hours straight, not including the many short runs since. He did think it sounded like air, but that it was leaking in, probably via the fuel filter seal, not leftover from the filter inspection debacle. However, reseating it made no difference either.

So I've got three theories, and I'm looking for people who know far more than me to point me in the right direction.
1/ Air is leaking in via the fuel filter seal.
2/ Removing the viscous old oil and replacing it with shiny new synthetic with its lovely detergents cleaned off the deposits that were the only thing maintaining compression, and now a lack of compression is making for hard starting (remember this thing has no glow plugs).
3/ The little diaphragms in the fuel pump are shot (evidence would be my inability to get more than a drop of fuel per wiggle of the lever) and that, combined with a partial case of cause #1 or cause #2, means I don't get enough fuel to start.

Any and all help will be deeply appreciated.
Sep 15, 2009
S2 9.2a Fairhope Al
when you start it cold set the throttle at 2/3 throttle and it will do better


Apr 22, 2009
Ontario 32 Pender Harbour
A few things I learned about Yanmars:
- they're supposed to be started with lots of throttle (ie NOT at idle)
- They'll smoke when they first start - it's just the cylinders warming up. Diesel doesn't run well when it's cold and they don't have glow-plugs
- They're a BITCH to bleed! My 2GMF sucked air after hitting a big wake. I gave up after an hr when my back went out. The mechanic took over 2 hrs to get the air out.

But you say it's run for hours, so it's PROBABLY not air (check: the return should go back to the tank, NOT to the filter). But you may be right that there's a small leak somewhere that's putting in a bit of air.

My Wild Guess: air in the fuel line. Somehow...
Jan 22, 2008
Hunter 34 Alameda CA
Eventually the copper washers on the fuel lines in and out of the secondary fuel filters crush. Air gets in there. The mechanical pump takes getting used to. You push it down until it stops (which feels really easy). Then push harder and it moves some more. That's the place where it pumps a shot of fuel. Like you, I have a squeeze bulb. Search the archives and you'll find where I mountd mine. It makes bleeding much easier. Mine takes a little longer to start in the winter but is quick in the summer. I use a bit of throttle too as suggested above for a quicker start. The fuel pump has a weep hole in it that leaks when the diaphram is shot. If you see no fuel in the bottom of the engine bay and it runs good the pump is probably good.
May 24, 2004
CC 30 South Florida
It sounds to me like your 30 year old engine has lost some compression. Diesel engines burn fuel by the heat created by cylinder compression. For start up Yanmar engines rely on the compression build up by a number of consecutive revolutions as produced by the starter motor. As the new oil is at its thinnest when the engine is cold it may requires more revolutions to build up compression. By opening the throttle lever to 2/3 you can speed up the cranking speed and the compression build up process. Make sure your batteries are fully charged.
Nov 6, 2006
Hunter 34 Mandeville Louisiana
I agree with Allan; those darn banjo washers and fittings can leak a little bit of air when the engine is stopped, This without leaking diesel while underway. Makes the engine tough to start but once you start it and run a while, it'll start again for a while.. then after a day or so won't start easily and may require more bleeding. Tighten all the phillips headed screws around the pump diaphragm first. seems hard, but if you use a 90 degree driver, it is not bad. Ya might consider changing the banjo washers.. Our local Yanmar guy found some banjo washers that have a coating of neoprene and are the best ever ! I think they are made for Izuzu diesels.. I bought a couple of the smaller diameter ones and they are great! I don't recall part numbers, but ya may be able to find a reference by searching this site.
Some folks anneal the washers to soften them and you might try that..
Be very careful that ya don't strip out the banjo fitting !
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Jan 13, 2015
Hunter 34 Deep Bay, BC
I was just looking over old threads and realized I never closed the loop on this one.
The answer was the fuel pump. The original mechanical pump was shot. My Yanmar mechanic said it was far easier and more effective to add an electric fuel pump than to replace the existing mechanical one, so I did that. I wired it so that when the key is energized, the pump is on. From that point onward the engine has started every time on about the second revolution, consistently, and regardless of weather. I've been a happy guy ever since. It also makes it really easy to bleed air out; open the bleed, turn on the key for a couple of seconds, and it's done.